decachlorotetrasilane

Toxicity to microorganisms

Description of key information

Toxicity to microorganisms: 3 hour EC50 > 100 mg/l (OECD 209), read-across from an analogous substance.

Additional information

There are no reliable microorganism toxicity data available for decachlorotetrasilane (CAS 113763-19-4), therefore good quality data for the analogous substance, tetraethyl orthosilicate (CAS 78-10-4), have been read across. Both substances hydrolyse rapidly to monosilicic acid. Tetraethylorthosilicate hydrolyses with a half-life of 4.4 hours at pH7 25°C to give monosilicic acid and ethanol.

Decachlorotetrasilane hydrolyses with a half-life of approximately 5 seconds (as a worst case) at 25°C and pH 7 (analogue read-across). This half-life relates to degradation of the parent substance to give decahydroxytetrasilane and HCl.

The Si-Si bonds may also react to produce monosilicic acid.

Monosilicic acid exists only in dilute aqueous solutions and readily condenses at concentrations above approximately 100 -150 mg/l as SiO₂ to give a dynamic equilibrium between monomer, oligomers and insoluble amorphous polysilicic acid.

Decahydroxytetrasilane is also likely to form condensation products (polyhydroxy-polysilanes).

Decachlorotetrasilane (CAS 13465-77-5) and tetraethyl orthosilicate (CAS 78-10-4) are within an analogue group of substances within which, in general, there is no evidence of significant toxicity to microorganisms.

This analogue group for the toxicity to microorganisms endpoint consists of a number of sub-classes of substances (based on structure/functionality) with different main codes (used to represent the different sub-classes). However, read-across is carried out between substances with the same main code in most cases.

The table below presents microorganism toxicity data for substances within the sub-class I-4 of Silicon without carbon attached (“(Poly)silicic acid producers”). Decachlorotetrasilane (CAS 13465-77-5) and tetraethyl orthosilicate (CAS 78-10-4) are members of this sub-class.

It is therefore considered valid to read-across the results for tetraethylorthosilicate (CAS 78-10-4) to fill the data gap for the registered substance. Additional information is given in a supporting report (PFA, 2013j) attached in Section 13 of the IUCLID 5 dossier.

Table: Microorganism toxicity data for substances within the relevant sub-class I-4 of chemicals

CAS	Name	Main code	Result: E(I)C50 (mg/l)	Result: NOEC (Or EC10/EC20) (mg/l)	Guideline	Test method	Species	Duration	Klimisch code
000078-10-4	Tetraethyl orthosilicate	I-4-Q	>100 mg/l		OECD 209	ASRI		3hr	1
068412-37-3	Silicic acid (H4SiO4), tetraethyl ester, hydrolyzed	I-4-Q		EC10 >2 ml/l	Huls AG method	oxygen consumption	P. putida	5 h	2

In the study with tetraethyl orthosilicate (CAS 78-10-4), an activated sludge respiration inhibition (ASRI) 3 hour EC₅₀ value of >100 mg/l was reported.

The study is considered to be reliability 1 (reliable without restrictions); the read across of the result is considered to be reliability 2 (reliable with restrictions). The hydrolysis rate for tetraethyl orthosilicate is 4.4 hours at pH7 and 25°C, and so it is likely to hydrolyse to monosilicic acid to a lesser extent than decachlorotetrasilane would hydrolyse over the timescale of the ASRI test.Therefore, microorganisms toxicity data for Silicic acid (H4SiO4), tetraethyl ester, hydrolysed (CAS 68412-37-3), which is a member of the same sub-class I-4 of Silicon without carbon attached (“(Poly)silicic acid producers”), has been read-across as supporting data. There is no evidence of significant toxicity to microorganisms in any of these studies, and read-across is considered valid. Microorganism toxicity data from a non-standard oxygen consumption study with Pseudomonas putida are available for Silicic acid (H4SiO4), tetraethyl ester, hydrolysed. A 5.42 hour EC₁₀ of >2 ml/l for toxicity to Pseudomonas putida was determined for silicic acid (H4SiO4), tetraethyl ester, hydrolysed in a reliable study conducted according to generally accepted scientific standards and described in sufficient detail, and was carried out in compliance with GLP, with acceptable restrictions. The restrictions were that a reference substance was not used to check the sensitivity of the microorganisms, and an emulsifier was used which could have affected the results. REFERENCE: PFA, 2013j, Peter Fisk Associates, STP Microorganism toxicity Main Analogue Group report, PFA.300.003.006.